1. Field of the Invention
The present invention relates to a semiconductor element typically a thin film transistor (TFT), using particularly a crystalline semiconductor film as a semiconductor layer including a channel forming region, a source region, and a drain region. Also, the present invention relates to a semiconductor device using such a TFT as a driver circuit or a switching element of a pixel (particularly, a liquid crystal display device or a light emitting device) and a manufacturing technique thereof. Further, the present invention particularly relates to a semiconductor device having a structure in which a light shielding property is improved and a manufacturing technique thereof.
2. Description of the Related Art
In recent years, a liquid crystal projector in which characteristics such as miniaturization and weight reduction are improved has been used in various situations. In response to that, competition of development for providing a liquid crystal projector having a smaller size and lighter weight is intensified. The liquid crystal projector is constructed so as to project an image and the like displayed on a liquid crystal display device and the performance of the liquid crystal projector is greatly influenced by a display quality of the liquid crystal display device.
As to the liquid crystal display device, the mainstream is one in which liquid crystal is sealed between a substrate in which a TFT and a pixel electrode are formed (hereinafter referred to as a TFT substrate) and a substrate in which a counter electrode is formed (hereinafter referred to as a counter substrate) and an alignment of the liquid crystal is controlled by an electric field produced between the pixel electrode and the counter electrode to display an image.
In recent years, an active matrix type liquid crystal display device (liquid crystal panel) having several million pixels in a pixel portion is greatly used as a liquid crystal display device. In such a liquid crystal panel, a TFT is provided in each of pixels as a switching element for providing a potential to each of pixels and a pixel electrode is provided in each TFT. When the TFT is turned on, the potential of the pixel electrode is set. When the TFT is turned off, the potential of the pixel electrode is kept by charges stored in a storage capacitor element (hereinafter referred to as a storage capacitor).
When the potential of the pixel electrode is changed while the TFT is in an off state, a display quality is deteriorated. Thus, it is required for an active matrix type TFT substrate that a leak current of the TFT is suppressed, a sufficient storage capacitance is obtained for each of pixels, and the amount of charges stored in the storage capacitor is sufficiently larger than that lost by a leak current.
Also, in the case of a transmission type liquid crystal panel, in order to increase the intensity, it is necessary to increase an occupying ratio of an opening portion, that is, a region which is intended to control display in a pixel (for example, a region through which light is transmitted and which contributes to display in the case of a transmission type display device, a region from which light is reflected and which contributes to display in the case of a reflection type display device, a region in which an organic light emitting layer sandwiched by electrodes emits light and which contributes to display in the case of a display device using an organic light emitting element, or the like).
Incidentally, in the case where the above-mentioned liquid crystal panel (in particular, a transmission type liquid crystal panel) is used for a liquid crystal projector, when light is incident into the semiconductor layer of a TFT, since a leak current due to photo-excitation (hereinafter referred to as a photo leak current) is caused, it has an adverse affect on display. Thus, a light shielding layer is provided in the liquid crystal panel. For example, when a light source of the projector is located in a counter substrate side, the light shielding layer is formed between a pixel electrode and a TFT to block light from the light source, or the light shielding layer is formed between a substrate and a semiconductor layer to block light reflected from a projection lens or the like. Also, according to Japanese Patent Application Laid-Open No. 2000-164875, a concave portion is provided in a substrate and a lower light shielding film is formed on the entire inner wall surface of the concave portion. Thus, the channel forming region of a TFT is formed so as to be buried in the concave portion. Also, an upper light shielding film is formed together.
However, according to the structure disclosed in the above publication, unevenness is formed near a TFT on which various wirings are concentrated. Thus, a possibility of reducing a yield is high because, at the time of wiring formation, a short circuit and a break of wirings are easily caused, or the wirings are easily deteriorated by the concentration of an electric field.
Also, according to the structure disclosed in the above publication, a gap is present between the upper light shielding film and the lower light shielding film. Thus, in the case of such a structure, there is also a possibility that a photo leak current by stray light is caused. Further, since the concave portion is provided in the substrate, there is a possibility that the mechanical strength of the substrate is reduced.
In the case of a projector for which a high intensity and a high definition are required, first, the intensity of a lamp used as a light source is increased to increase a display brightness. Second, the number of pixels in a panel used for an optical system is increased to obtain a higher definition. However, in the conventional methods of forming the light shielding film between the pixel electrode and the TFT and of forming the light shielding film between the substrate and the semiconductor layer, there is a problem that light diffracted by end portions of the light shielding film is incident into the semiconductor layer to cause a photo leak current.
Further, with increasing the intensity of the light source, an adverse affect on the TFT by the diffracted cannot be neglected any longer.
Also, when a thin insulating film is used for isolating the light shielding film and the TFT, the intensity of the diffracted light in the position of the TFT call be reduced to a negligible extent. However, when the insulating film is made thinner, a parasitic capacitance produced between the TFT and the insulating film is increased. Therefore, a problem occurs in that an operation of the TFT is influenced by a potential of the light shielding film.
Also, when a width of the light shielding film is expanded, a problem that diffracted light is incident into the TFT can be solved. However, it is natural to reduce an aperture ratio. In addition, since the requirement for a high definition of display is satisfied by increasing the number of pixels, a size of respective pixels is decreased. Thus, a reduction in an aperture ratio due to the expansion of the width of the light shielding film and a reduction in brightness accompanied by such a reduction become a large problem.
Also, only when the width of the light shielding film is expanded, a problem that stray light produced by unintended scattering in an interlayer insulating film is incident into the TFT (in particular, the semiconductor layer) cannot be solved. With increasing the intensity of the light source as described above, the influence of the stray light also cannot be neglected.
Also, in a TFT including an active layer having a crystalline structure, which has been actively used because of its high field effect mobility and the like, a photo leak current tends to increase as compared with a TFT including an amorphous semiconductor layer. If the TFT have no sufficient storage capacitance, stored charges are decreased by the leak current to change the amount of light to be transmitted, which becomes a cause for reducing a contrast in image display. Thus, it is necessary to form a storage capacitor element capable of securing a sufficient capacitance in a liquid crystal panel.
However, when an area of the storage capacitor is expanded in two dimensions to secure the sufficient capacitance, an occupying ratio of the storage capacitor element to an area of a pixel is increased to reduce an aperture ratio.
Further, in order to improve a yield, it is necessary to use a structure in which a break of wiring and the like are not caused by unevenness due to the presence of the storage capacitor.